Many types of semiconductor devices are used in high power applications, requiring robust and reliable packaging. A high power package typically includes a semiconductor chip, a heat sink and a lead frame. The lead frame enables external electrical connections to be made to the semiconductor chip while electrically isolating the connections from the heat sink. The lead frame is typically made of a ceramic material such as alumina. The lead frame is conventionally brazed to the heat sink at a relatively high temperature, e.g. around 800° or higher so that the interface between the heat sink and the lead frame can withstand extreme temperature conditions during use in the field.
Ideally, the heat sink would be made from essentially all copper which has a high thermal conductivity (385 W/mK). Such a heat sink would enable the package to efficiently dissipate large amounts of waste heat energy generated by the semiconductor chip. However, high brazing temperatures preclude the use of a mostly copper heat sink because copper has a CTE (coefficient of thermal expansion) of about 17 ppm and an alumina lead frame has a CTE of about 7 ppm. The CTE mismatch between a high-copper content heat sink and a ceramic lead frame would result in the heat sink expanding and contracting much more than the lead frame during the brazing process, resulting in heat sink bowing and damage to the package.
For this reason, conventional high power packages typically use a heat sink formed from a metal matrix composite material such as CuMoCu, CuTg, WCu, etc. Metal matrix composite materials have a lower CTE than copper. The lower CTE of the metal matrix composite material more closely matches the CTE of a ceramic substrate, reducing the amount of stress-induced damage caused during brazing. However, metal matrix composite materials such as CuMoCu, CuTg, WCu and the like have a much lower thermal conductivity than copper. For example, CuMoCu has a thermal conductivity of about 260 W/mK and CuTg has an even lower thermal conductivity of about 180 W/mK, both significantly lower than that of copper (385 W/mK). Such low thermal conductivity values degrade the overall thermal performance of the package which can be catastrophic for high power applications.